Particularly in the case of medical imaging for the purpose of imaging the heart and the blood vessels near the heart, there is a general problem that as a consequence of the heartbeat, the examination area to be recorded is subject to a continuous periodic movement as a result of which tomographic images are comparable with one another only whenever they have been recorded in relation to the same phases of a cardiac cycle. This circumstance is problematic particularly in the case of computed tomography examinations in the case of which the tomographic images to be produced are calculated by back projection of a multiplicity of projections acquired from different projection angles. The back projection is generally successful and free from interference when the basic projections image a substantially identical phase of the cardiac cycle. A displacement, caused by cardiac arrhythmias, of the scanning with reference to the phase is expressed in movement artifacts in the resulting tomographic image.
In order to enable a reconstruction of a tomographic image representing the heart with small movement artifacts, projections can be obtained in relation to one and the same phase from a multiplicity of different projection directions by evaluating an ECG signal derived from the patient. There are two methods for ECG controlled acquisition of projections that can be distinguished in principle by approach.
One possibility resides in acquiring projections during the entire cyclic duration of the heart movement, and storing them together with the ECG signal. The reconstruction of a tomographic image is performed following on from the data acquisition, projections relating to defined phases retrospectively being selected using the ECG signal. One advantage of this method consists in that it is possible to display arbitrary movement phases of the heart by suitable selection of the data intervals. A precondition for such a retrospective gating in the case of the reconstruction of tomographic images is that the patient be irradiated during the entire scanning with a full X-ray dose, the result being the application of a substantially higher X-ray dose than necessary.
A further possibility resides in carrying out sequential scans triggered prospectively by the ECG in order to minimize the radiation dose during cardiac computed tomography. Such a method is described, for example, in DE 10 2006 060 482 A1. In the case of such sequential scanning, the recording system is moved relative to the examined object to various z-positions along the z-axis, and projections are respectively prepared at the relevant z-position, the time window in which the projections are produced being defined as a function of the ECG signal. For example, it is possible to define a specific time window by a starting point and an end point that are determined relative to a previously measured last R-wave in the ECG. The data recording is mostly performed in this case in an accurately defined time window in the area of the end diastole, in order to display the coronary vessels in a fashion free from movement.
For functional cardiac imaging, in particular for the purpose of determining the ejection fraction, that is to say the proportion of the blood volume ejected during the contraction of the heart to the total volume of the ventricle, there is additionally the need also to prepare an image in the phase of the maximum contraction, that is to say at the instant of the end systole.